18 Pa.C.S. § 4302: Incest.

(a) General rule.--Except as provided under subsection (b), a person is guilty of incest, a felony of the second degree, if that person knowingly marries or cohabits or has sexual intercourse with an ancestor or descendant, a brother or sister of the whole or half blood or an uncle, aunt, nephew or niece of the whole blood.

(b) Incest of a minor.--A person is guilty of incest of a minor, a felony of the second degree, if that person knowingly marries, cohabits with or has sexual intercourse with a complainant who is an ancestor or descendant, a brother or sister of the whole or half blood or an uncle, aunt, nephew or niece of the whole blood and:

(1) is under the age of 13 years; or

(2) is 13 to 18 years of age and the person is four or more years older than the complainant.

(c) Relationships.--The relationships referred to in this section include blood relationships without regard to legitimacy, and relationship of parent and child by adoption.